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Lab Activity 6A) Peroxide Value DeterminationB) Formation of AcroleinC) Saponification

IUG, 2015Dr. Tarek Zaida

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A) Peroxide Value Determination

• Peroxide value is the concentration of (-O-O-) groups in edible oils,

• It is a measurement of the decomposition of the product

• In many countries, official standards specify a maximum peroxide number beyond which the oil is unfit for human consumption.

• Formation of peroxide during storage of oil or fat may occur after few weeks to several months according to the conditions of storage

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• The peroxide number is therefore measured by oil manufacturers during production and after storage to check its preservation.

• International standards use a redox titration in non aqueous media, ‐

• Results are generally expressed in μg of peroxide (or active oxygen) per gram of product

• But mmoles/kg or meq of O2/kg are also used.

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• Peroxide value is determined volumetrically• Reaction of KI in acid solution with the bound

oxygen, followed by titration of the liberated I2 with sodium thiosulfate.

• Chloroform is used as a solvent.• Fresh oil has peroxide value below 10 meq/kg. • Rancid taste often begins to develop when the

peroxide value is between 20 & 40 meq/kg

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• Peroxide number determination involves a two step redox reaction‐ :

1) Reaction of peroxide group with an excess of iodide ion according to:

R O O R + 2I‐ ‐ ‐ ‐ + 2H+→2ROH + I2

2) Titration of I2 with Na2S2O3 solution 0.002N, according to:

I2+ 2S2O32‐→2I‐+ S4O6

2‐

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Materials

• 250 ml Erlenmeyer or volumetric flask• Chloroform• Fresh saturated aqueous KI solution (15g /10 ml

H2O) store in dark• Glacial acetic acid • 0.1 M Thiosulfate• Starch• Oil sample

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Procedure

1. Weigh 1 to 4 g oil sample into 250 ml flask2. Add 10 ml chloroform, dissolve the oil by swirling3. Add 15 ml of glacial acetic acid4. Add 1 ml of a fresh saturated aqueous KI solution5. stopper the flask, shake for 1 min and place the flask for 5

min in dark6. Add about 75 ml distilled, mix and titrate (Vml) the formed

I2 with 0.002 N solution of thiosulfate using starch solution (1%) as indicator.

7. Carry out a blank titration (V0ml) which should not exceed 0.5 ml of 0.002 N thiosulfate solution

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Calculation

• Peroxide value = (V- V0) T x 1000 meq/Kg

• Where T is the exact molarity of thiosulfate solution

weight of sample (g)

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B) Formation of Acrolein

• Reagents:• Olive oil, melted butter, potassium bisulfate

KHSO4 , Schiff’s reagent • Schiff's reagent is a solution that will combine

chemically with aldehydes to form a bright red product.

Glycerol KHSO4 acrolein + 2H2Oheat

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Procedure

1. Place about 1 g powdered KHSO4 in a clean

test tube.2. add 3 – 4 drops of olive oil (0.5 g melted

butter) on the salt and heat.3. Note the irritating odor of acrolein aldehyde

or aldehyde which will color a filter paper moistened with Schiff’s reagent bright red

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C) Saponification (Formation of a soluble soap & insoluble soap)

• ReagentsOlive oil,cow fat,5% KOH solt.2% MgSO4 solt.

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Procedure

1. add 10 ml of olive oil or 10 g of cow fat in a 250 ml beaker

2. add 50 ml KOH soln.3. add 150 ml dist. Water4. Hydrolyze the lipids by heating the beaker

nearly the boiling point for 3 – 5 min5. Transfer a few amount of the beaker content

into about 30 ml of dist. Water.

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6. Observe if any saponificaton has occurred (indicated by the complete solubility of the solution when fall into a dist. Water).

7. To form insoluble soap, add a few mls of 2% MgSO4 solution to the soap solution, while precipitate indicate formation of magnesium salts of fatty acids (insoluble soap).